This invention relates to semiconductor photodetectors, and more particularly to a heterojunction photodiode device of improved wavelength-selectivity.
To fully utilize the bandwidth capacity of optical fibers, wavelength multiplexing may be used, in which multiple carrier wavelengths are generated by LED or laser sources and the optical energy of all the sources is coupled to a single fiber trunk for transmission.
At the receiver, several carrier wavelengths can be separated by a variety of techniques. One such technique makes use of broadband optical detectors coupled with wavelength-selective components such as lenses, prisms, gratings, and interference filters. Such a technique has the disadvantage of requiring precise tuning and orientation of its multiple components.
It would be highly desirable if the detection and demultiplexing of multiple-wavelength optical signals could be performed by a semiconductor photodetector, which would be simpler in design than the prior art devices, and whose wavelength response would be inherent in its component materials and thus not depend on delicate mechanical adjustments or manufacturing steps.
Such a detector should have high selectivity, thus maximizing the number of channels that a single optical fiber can accommodate without objectionable crosstalk. It should operate in the near-infrared region (about 0.8 to 1.6 .mu.m) most important in optical-fiber communications, and have high response speed to further improve signal bandwidth. The device further should have high sensitivity, low noise, and low biasing voltage or current requirements, and be both compact and reliable. These characteristics, and particularly selectivity and response speed, are directly related to optimizing the information-carrying capacity of a given optical fiber.
It has been attempted to achieve such results with a heterojunction photodiode, in which a window layer of one conductivity type having a wide bandgap (say Eg.sub.2) forms a PN junction with an active layer of the opposite conductivity type having a narrow bandgap (say Eg.sub.1).
Photons whose energy exceeds Eg.sub.2 are absorbed as they enter the window layer. Photons with energy less than Eg.sub.2 but greater than Eg.sub.1 pass through the window layer and are absorbed in the vicinity of the junction, and preferably in the depletion region thereof, as they enter the active layer, generating carriers which constitute the desired photocurrent. No photoresponse occurs to photons whose energy is less than Eg.sub.1, these passing entirely through the detector.
The photodiode thus responds primarily to photons whose energy E is in the range Eg.sub.2 &gt;E&gt;Eg.sub.1. In terms of wavelength, response is limited to .lambda..sub.1 &gt;.lambda.&gt;.lambda..sub.2 where .lambda..sub.1 and .lambda..sub.2 correspond to Eg.sub.1 and Eg.sub.2, respectively.
As photon energy exceeds the Eg.sub.1 level, photoresponse increases rapidly due to the exponential absorption edge characteristic of direct gap semiconductors. Photoresponse can approach unity quantum efficiency (electron-hole pairs generated per photon) until the energy reaches Eg.sub.2.
Quantum efficiency does not decrease rapidly to zero as E exceeds Eg.sub.2 in such a device, however, because high-energy carriers generated in the window layer diffuse to the junction and contribute a substantial short-wavelength diffusion tail to measured photoresponse. This lack of a sharp high-energy (short-wavelength) cutoff has an obvious deleterious effect on selectivity. Moreover, the substantial phase difference between the current due to diffused carriers and that due to those that cross the junction by drift has a deleterious effect on response time, and therefore places an upper limit on the range of usable signal frequencies.